1. Field of the Invention
The present invention relates to an image forming apparatus to form an image by developing a latent image formed on an image carrier by a development apparatus and transferring the developed developer image at a transferred position.
More specifically the present invention relates to an image forming apparatus having the image carrier formed with the latent image, a developing device having a developing roller driven to rotate by a stepping motor and carrying a developer for developing the latent image, and a rotary development apparatus for supporting the developing device centering on a rotating shaft to move to rotate to a developing position opposed to the image carrier and constituted to form the image by developing the latent image formed at the image carrier at a developing position by the developing device and transferring the developed developer image to a medium at the transferring position.
2. Related Art
An image forming apparatus is arranged with a charging device for uniformly charging a photosensitive body along a rotational direction, an exposing device for forming an electrostatic latent image on the photosensitive body, a developing device for developing the electrostatic latent image by bringing a developing roller into contact with the photosensitive body, a transferring apparatus for transferring a toner image on the photosensitive body onto a transferring member, a cleaner for cleaning a surface of the photosensitive body after having been transferred therewith at an outer periphery of the photosensitive body which is an image carrier. Further, the toner image is formed by developing the electrostatic latent image formed on the photosensitive body by the exposing device by the developing device, the formed toner image is transferred onto the transferring member, and the toner remaining on the photosensitive body after having been transferred therewith is removed from above the photosensitive body by the cleaner to clean.
In the case of an image forming apparatus for forming a full color image, color toner images formed on the photosensitive body by the respective developing device of yellow Y, magenta M, cyan C, and black K are successively transferred onto an intermediate transferring medium to overlap colors. Therefore, there are a rotary type and a tandem type, in the types, the image forming apparatus of the rotary type is provided with a rotary development apparatus attachably and detachably mounted with a plurality of developing device (developing cartridges) radially centering on a rotating shaft of a rotary frame for developing the electrostatic latent image formed on the surface of the photosensitive body to the toner image by successively moving to rotate the developing device of respective colors of the rotary development apparatus to a developing position opposed to the surface of the photosensitive body by operating to switch the developing colors of the rotary development apparatus. In the color image forming apparatus having the rotary development apparatus, there is known a system of butting a rolling member arranged coaxially with the developing roller to the photosensitive body by urging the developing device in a direction of the photosensitive body to position relative to the photosensitive body for ensuring a proper gap between the photosensitive body and the developing roller (developer carrier) (refer to, for example, JP-A-2003-66801 and JP-A-2002-82532).
The rotary development apparatus is provided with the developing position, an interchanging position, and a standby position of the respective developing device for moving to rotate and stop, as a method of successively positioning to lock the respective developing device at the predetermined positions, it is the simplest to utilize a holding force of a drive motor for rotating the rotary, however, in consideration of power consumption, holding performance and the like of the drive motor in holding, it is preferable to position the rotary by some mechanical means without depending on the holding force of the drive motor. Hence, it has been proposed to mechanically position the rotary by engaging a lever provided at a main body of the image forming apparatus with a portion of the rotary (refer to, for example, JP-A-2002-287461). The lever is provided movably between an escaping position which does not position the rotary and a positioning position (constraining position) for positioning a rotary by engaging with the rotary and is engaged with a portion of a rotary frame at the positioning position.
According to the color image forming apparatus having the rotary development apparatus of the above-described background art, when the full color image is formed, development and transcription are similarly repeated while successively switching a plurality of developing device by operating to switch developing colors of the rotary development apparatus to overlap the plurality of toner images, however, when a single color image is formed, the toner image is formed repeatedly by making a predetermined one of the developing device opposed to the photosensitive body. That is, in the case of the full color image, rotation of the developing roller is stopped when the developing device are switched by operating to switch the developing colors of the rotary development apparatus, however, for example, when an image of only black color is continuously formed, it is not necessary to switch the developers and it is not also necessary to stop to rotate the developing roller. Therefore, even when development is not actually carried out, for example, the developing roller is opposed to an interval between the latent images of the photosensitive body, so-to-speak an interval between sheets, the developing roller continues rotating while carrying the toner and therefore, the toner is wastefully consumed to contaminate inside of the apparatus.
Further, a member for restricting a layer thickness of the toner is brought into contact with the developing roller, by rotating the developing roller, the layer thickness of the toner (developer) carried by the developing roller is restricted, and electric charge is applied to the toner by friction. Therefore, while the developing roller is rotating, the developing roller and the layer thickness restricting member and the toner, or the toners are slid to produce friction and are considerably worn.
Therefore, when the developing roller continues to rotate during a time period of from printing on a first sheet to finishing printing on a final sheet, particularly including a time period during which development is not carried out as in the case of printing continuously a plurality of sheets of images by a single color toner, there poses a problem that there is a concern that the toner and the layer thickness restricting member are uselessly worn, proper electric charge is not applied to the toner, and image quality of the formed image is deteriorated. Further, since almost all of printing only by the single color toner is carried out by a toner in black color and therefore, there poses a problem that wear of the single color toner and wear of the member of the developing device is more progressed than that of toners of other colors and amounts of wearing members constituting the developing device significantly differ from each other among the developing device.
Moreover, in such the image forming apparatus, the developing roller brought into contact with the photosensitive body rotating in steady state is provided with a rotating mode and a stopping mode and is operated to switch from the rotating mode to the stopping mode and from the stopping mode to the rotating mode in a state of rotating the photosensitive body in steady state. As a method of driving the developing roller in this case, there is proposed a method in which at a vicinity at which a ratio of peripheral speeds of the developing roller to the photosensitive body is 1, an acceleration thereof is made to be larger than that of other speed region (refer to, for example, Japanese Patent Publication No. 2909319). This is for resolving a drawback that at the vicinity at which the ratio of the peripheral speeds is 1, the developer is uselessly adhered to the photosensitive body. Further, as other method, a stepping motor is decelerated to stop by a smooth deceleration profile (refer to, for example, JP-A-11-143296). This is for resolving a problem that a contact portion of a cleaning blade is turned up by reversely rotating the photosensitive body by being brought into a reversely rotating region relative to a rotational direction before being stopped by influence of inertia or the like in stopping the stepping motor.
However, according to the method of increasing the acceleration more than that of the other region at the vicinity at which the ratio of the peripheral speeds of the developing roller to the photosensitive body is 1, in the midst of acceleration or in the midst of deceleration, a load of the photosensitive body is rapidly varied, further, the load is also large since the ratio of the peripheral speeds is near to 1. Therefore, there poses a problem that a variation in the speed of the processing member rotating in steady state is increased and so-to-speak non-uniformity in rotation is brought about.
Similarly, according to the method of stopping the developing roller by the smooth deceleration profile, a change in the load of the photosensitive body at the vicinity at which the ratio of the peripheral speeds of the developing roller to the photosensitive body is 1 is increased. That is, when the ratio of the peripheral speeds becomes proximate to 1 from a state in which the peripheral speeds of the developing roller and the photosensitive body differ from each other to bring about considerable slippage, the slippage is reduced and the load of the photosensitive body is rapidly increased. Therefore, there poses the problem that the variation in the speed of the photosensitive body is increased and so-to-speak non-uniformity in rotation is brought about.
Even in the case in which the latent image formed on the photosensitive body is not being developed, when the latent image is being formed on the photosensitive body or when the toner image formed on the photosensitive body is being transferred, the non-uniformity in rotation of the photosensitive body constitutes a non-uniformity in the latent image or a non-uniformity in the transferred toner image and as a result, there poses a problem that image quality is deteriorated by bringing about a non-uniformity in the image.